The present invention relates to a printed wiring board having a shielding layer.
A printed wiring board having an electromagnetic wave shielding layer provided through an insulating layer is well known from Japanese Patent Laid-open No. 213,192/87. The printed wiring board 9 disclosed in this Japanese Patent Laid-open comprises, as shown in FIG. 3, an insulating sheet 1, printed wiring circuits 2 formed on both surfaces of the insulating sheet 1, an insulating layer 4 provided on a part of the printed wiring circuits 2, an electromagnetic wave shielding layer 5 provided on the insulating layer 4 and a part of the printed wiring circuit 2 and a solder resist layer 6 provided on the shielding layer 5.
In the forming portion of the printed wiring circuit 2 and the shielding layer 5 in the conventional printed wiring board 9 having the shielding layer 5, the insulating layer 4 is formed on the printed wiring circuit 2 by a screen printing process, and as shown in FIG. 3b, on a connecting land 20 to the shielding layer 5 in the printed wiring circuit 2, a relief angle portion 40 having the same diameter as that of the connecting land 20 (for example, 3 mm.phi.) is provided, and then a grounded circuit 5a is formed on the relief angle portion 40 together with the shielding layer simultaneously.
Between the relief angle portion 40 formed on the connecting land 20 and the insulating layer 4, however, uneveness corresponding to the thickness of the insulating layer 4 formed on the printed wiring circuit 2 is present, so that the thickness of outer periphery portion 5b of the grounded circuit 5a out of the shielding layer 5 formed by the screen printing process becomes thin compared with the thickness of the other shielding layer 5 and thus the shielding ink does not fill in or flow in the relief angle portion 40 smoothly, thereby causing an unevenness of the shielding layer and failing a formation of proper grounded circuit 5a.
At the relief angle portion 40, moreover, the grounded circuit 5a having a thickness more than necessary is formed (for example, if the thickness of shielding layer 5 is about 20 .mu.m, the grounded circuit 5a having a thickness of about 50 .mu.m is usually formed at the relief angle portion) while this grounded circuit 5a is commonly formed by the shielding paste, so that the more the thickness of coated film, the less the flexibility, and thus the grounded circuit becomes brittle (for example, a crack is caused in the grounded circuit 5a in the case of thermal expansion and shrinkage under solder dipping.)